This invention relates to the field of ceramics and particularly to colloidal methods of fabricating ceramics.
In order to fabricate reliable structural ceramics, it is necessary to use processing steps which achieve both chemical and microstructural uniformity in complex sintered shapes. Inhomogeneities and nonuniformities degrade the strength of ceramics, both in the brittle regime of fracture and in the viscous flow regime at high temperatures. In particular, agglomerates must be avoided in the manufacture of structural ceramics. Their presence in powders has a number of effects, including: (a) the formation during sintering of crack-like voids which result in lower strength, (b) an increase in the temperature required to sinter to final density, (c) a reduction in the attainable endpoint density from that theoretically available, and (d) a reduction in chemical homogeneity in multiphase ceramics.
The powders used to fabricate structural ceramics contain soft agglomerates which form spontaneously in dry powder due to Van der Waal forces or residual moisture, and hard agglomerates which are collections of sintered particles formed during the calcination step of powder manufacture.
Soft agglomerates can be broken down by dispersing the powder in a liquid. Because the volume percent (v/o) of the dispersed powder in the liquid is usually less than 20 v/o, the powder should be consolidated into a non-flowable shape by raising the solid content to more than 50 v/o. The consolidated shapes are dried and then sintered to form the desired structural component. However, all current consolidation routes are limited. For example, filtration exhibits parabolic rate kinetics, thus limiting the thickness of the consolidated layer that can be built up before the colloid becomes unstable (either sediments or flocs). This problem results in a density gradient within the consolidated layer. Other routes to consolidating dispersions such as centrifuging, electrophoresis, and evaporation, either differentiate with regard to the particles' mass, or electric charge, or are also limited by kinetics.
Hard agglomerates cannot be broken up by surfacants. The manufacturers of powder attempt to break up the hard agglomerates prior to sale by attrition milling. Although milling does reduce the size of most agglomerates, large agglomerates are still present in milled powders. Additionally, milling introduces contaminants into the powder.